Hydrothermal pretreatment for co-digestion of corn stover and sewage sludge: focusing on solids loading, phosphorus fate, and energy balance.

Corn stover and dewatered sewage sludge were co-digested after hydrothermal pretreatment. This study focused on temperature, residence time, and the often overlooked parameter of solids loading. The optimal condition was 180 °C for 1.5 h at 7.5% solids (designated A4), achieving a methane yield of 34.4 mL CH4/g VS, a 71% increase over the untreated control. Kinetic modeling confirmed that hydrothermal pretreatment significantly improved both methane potential and digestion kinetics. Phosphorus transformation analysis revealed that hydrothermal pretreatment promoted the hydrolysis of solid organic phosphorus to orthophosphate in the liquid phase. During anaerobic digestion, microorganisms partially re-assimilate this orthophosphate into organic phosphorus. The net energy balance was positive for all samples, reaching 1.77 kJ/g VS for the A4 condition. Microbial analysis showed that hydrothermal pretreatment selectively enriched hydrolytic and fermentative bacteria (notably Chloroflexi and Synergistetes) and increased the relative abundance of methanogenic Euryarchaeota to 98.6%. This enrichment established a syntrophic network that positively correlated with enhanced methane production. Optimizing solids loading is therefore critical for promoting the dynamic transformation and redistribution of phosphorus species between solid and liquid phases.